Electrolytic plating method and device for a wiring board

ABSTRACT

A plating bath which accommodates an insoluble node and a printed-circuit board, and a copper dissolved bath which supplies copper ions are arranged. The insoluble anode Is arranged as opposed to the printed-circuit board being a cathode, and a forward/reverse current is applied between both of the electrodes. Iron ions are added to a plating solution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrolytic plating method anddevice filling up a microvia hole formed on a wiring board with metalplating.

2. Description of the Related Art

For electric appliances such as a cellular phone, a video camera, anotebook computer, etc., it is demanded to mount high-densitycomponents. As an implementation of high-density mounting, a buildupboard on which a wiring layer and an insulation layer are sequentiallyformed, a printed-circuit board of an all-layer microvia type to whichwiring boards on which microvias are formed are attached with heat andpressure, etc. are proposed.

For a conventional buildup board, micro holes (microvia holes) areformed on an insulation layer, and the inner side and the bottom of theholes are metal-plated, so that wiring layers above and below theinsulation layer are electrically connected.

With this method, however, it is difficult to further form one microviahole on another, and to securely connect the holes in an electricmanner. Therefore, a land cannot be arranged on a microvia hole aftermicrovia holes are stacked. Due to such a restriction on a patterndesign, the whole of a pattern design cannot be made with an automaticwiring tool, and part of the design must be made manually. As a result,the time period required to design a printed-circuit board becomes long.

To overcome such a problem, a technique filling up microvia holes withelectrolytic plating is proposed. For example, Japanese Laid-open PatentPublication No. 8469 discloses the technique filling up microvia holesby performing electric metal plating with PR electrolysis after anelectroless metal film is formed.

However, with the plating method using PR electrolysis disclosed by theabove described publication, plating must be performed for a long time(for example, two hours or longer) to fill up microvia holes. Therefore,the manufacturing cost of a printed-circuit board increases, and it isdifficult to use a printed-circuit board on a mass-production level.Additionally, attempts are made to improve the density of an electriccurrent in order to shorten a plating time. However, problems such thata void occurs during plating, or a plated surface becomes rough occur.

Furthermore, to solve the problems occurring when a soluble anode isused, for example, Japanese Laid-open Patent Publication No. 507106discloses a metal plating method using an insoluble anode and a platingsolution to which an oxidization-reduction compound is added.

The above described invention assumes electrolytic plating using adirect current power source, and does not present a plating method forfilling up microvia holes on a printed-circuit board for a short time.

SUMMARY OF THE INVENTION

An object of the present invention aims at filling up microvia holes ona printed-circuit board for a short time.

According to the present invention, a printed-circuit board is used asone pole and an insoluble electrode is used as the other, andelectrolytic plating is performed by applying a forward/reverse currentwith the use of a metal plating solution including iron ions by 0.1gram/liter or more, so that microvia holes formed on a printed-circuitboard are filled up with metal plating.

According to the present invention, electrolytic plating is performed byapplying a forward/reverse current with the use of a metal platingsolution including iron ions by 0.1 gram/liter or more, whereby microviaholes can be filled up for a time shorter than a conventional method,and a metal film having a smooth surface characteristic can be formed.As a result, microvias which electrically connect wiring layers aboveand below an insulation layer can be formed for a short time, therebysignificantly reducing the manufacturing cost of a multi-layerprinted-circuit board.

As an electrolytic plating method, for example, a pulse reverseelectrolytic method applying a forward/reverse pulsed current isavailable.

Additionally, a plating solution may be stirred to flow in parallel withthe surface to be plated of a printed-circuit board. At this time, theflow quantity of the plating solution may be controlled depending on thediameter or the depth of a microvia hole.

With the above described configuration, the deposit speed of metalplating on the surface of a printed-circuit board and that of metalplating within a microvia hole can be suitably controlled. Consequently,a deep microvia hole with a short diameter can be filled up withoutcausing a void, etc. within the hole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 explains the manufacturing process of a multi-layerprinted-circuit board;

FIG. 2 explains an electrolytic plating method according to a preferredembodiment;

FIG. 3 shows the flow of a plating solution;

FIG. 4 shows the waveform of a plating current;

FIG. 5 shows the plating conditions of Samples;

FIG. 6 shows the results of measuring the degree of roughness ofsurfaces in Samples;

FIGS. 7A and 7B show the cross-sectional view of a microvia according tothe preferred embodiment; and

FIG. 8 shows the cross-sectional view of a microvia when being platedwith a plating solution that does not include iron ions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment according to the present inventionwill be described by referencing drawings. First of all, themanufacturing process of a multi-layer printed-circuit board isexplained by referencing FIG. 1.

A wiring pattern (wiring layer) 11′ is formed by etching a copper foil(conductor layer) stacked onto a core resin 12 such as glass epoxy, etc.(process steps (1) and (2) in FIG. 1). Next, an insulation layer 13 isformed on the wiring pattern (process step (3) in FIG. 1). Holes aredrilled in the insulation layer 13 with laser, etc., so that microviaholes 14 are formed (process step (4) in FIG. 1). Next, a copper platedlayer 15 is formed with electrolytic plating, etc. to fill up themicrovia holes (process step (5) in FIG. 1). In the plating process step(5) in FIG. 1, after a thin conductor layer is formed with chemicalplating, etc. on the wiring pattern 11′ at the bottom of the insulationlayer 13 and the microvia holes 14, the microvia holes 14 are filled upwith pulse reverse electrolytic plating to form the copper plated layer15. Then, a wiring pattern 15′ is formed by etching the copper platedlayer 15 (process step (6) in FIG. 1). As a result, the wiring patterns11′ and 15′ above and below the insulation layer 13 can be electricallyconnected.

The technique filling up microvia holes with pulse reverse electrolyticplating is recited, for example, in “Gist of the 100th Lecture (held onOct. 6 and 7, 1999) by Surface Finishing Society of Japan.

Next, an electrolytic plating method for a printed-circuit boardaccording to this preferred embodiment will be explained by referencingFIG. 2.

A plating bath 21 is composed of insoluble anodes 22, a cathode 23 beinga printed-circuit board, a power source 24 for applying aforward/reverse current between the electrodes, and a copper platingsolution including iron ions. To widen the surface areas of theelectrodes, a multi-aperture electrode such as an expanded metal, etc.is used as each of the insoluble anodes 22.

Besides, copper dissolved baths 25 are arranged to supply copper ions tothe plating bath 21, and a solution within the copper dissolved baths 25and the plating solution within the plating bath 21 are circulated by acirculation pump 26.

According to this preferred embodiment, an iron ion “Fe²⁺” is added tothe plating solution, so that “Fe³⁺+e” is generated from “Fe²⁺” in theproximity of the insoluble anodes 22 as shown in FIG. 2.

In the copper dissolved baths 25, “Cu²⁺” and “Fe²⁺” are generated by thedissolution reaction between the copper material within the copperdissolved baths 25 and “Fe³⁺” which is generated by each of theinsoluble anodes 22 and carried to the copper dissolved baths 25.

At the cathode 23, Cu is deposited from “Cu²⁺” which is carried from thecopper dissolved baths 25, so that a copper plated layer is formed onthe printed-circuit board. At the same time, “Fe²⁺” is produced from“Fe³⁺+e” which is generated by the insoluble anodes 22.

Namely, “Fe³⁺” is generated from the iron ion “Fe²⁺” included in theplating solution as a result of the electrolytic reaction of theinsoluble anodes 22, and “Cu²⁺” and “Fe²⁺” are generated by “Fe³⁺” andthe copper material within the copper dissolved baths 25. Therefore, thecopper ion “Cu²⁺” and the iron ion “Fe²⁺”, which is added to the platingsolution and consumed by the reaction of the insoluble anodes 22continue to be supplied from the copper dissolved baths 25.

FIG. 3 schematically shows the flow of the plating solution within theplating bath 21 according to this preferred embodiment.

The cathode (printed-circuit board) 23 is arranged in the middle of theplating bath 21, and the two insoluble anodes 22 in a meshed state arearranged as opposed to the printed-circuit board 23. The platingsolution is circulated by the circulation pump 26 in the right directionof FIG. 3. That is, the plating solution is circulated to flow inparallel to the surface to be plated by a predetermined flow quantity.By making the plating solution flow in parallel to the surface to beplated of the printed-circuit board 23, microvia holes are completelyfilled up, and a plated layer having a suitable film thickness can beformed. This can be considered to be implemented, because the depositspeed of copper on the surface of the printed-circuit board 23 and thatof copper within the microvia holes can be adjusted by making theplating solution flow, for example, in parallel to the surface of theprinted-circuit board 23 to control the amount of “Fe3+” existing on thesurface of the printed-circuit board 23.

Described next are plating conditions and evaluation results of platingwhen a microvia hole having a depth of 50 μm, which is formed on aninsulation layer, is filled up with the plating method according to thispreferred embodiment.

The fundamental composition of the plating solution used in thispreferred embodiment is as follows:

-   -   copper sulfate·5 hydrates: 235.7 g/liter (L)    -   sulfuric acid: 60 g/L    -   organic additive (surface active agent such as Impulse Leveler        provided by Atotec Co., Ltd.)    -   organic additive (brightener such as Impulse Brightener provided        by Atotec Co., Ltd.)    -   chloric ion: 40 mg/L    -   iron ion: 15 g/L (or 0.1 g/L)

In this preferred embodiment, pulse reverse electrolytic plating isperformed by applying a forward/reverse pulsed current to theelectrodes. The plating current applied to both of the electrodes is aforward/reverse pulsed current having a forward current duration T1 thatis 40 ms, and a reverse current duration T2 that is 2 ms, as shown inFIG. 4. Furthermore, the average current density of the cathode is setto 3A/dm².

FIG. 5 shows the plating conditions of Samples 1 through 3 of aprinted-circuit board for which pulse reverse electrolytic plating isperformed, namely, the amount of iron ions included in the platingsolution, the average current density, a plating time, and the thicknessof a plated layer.

Sample 1 indicates the plating performed for 33.3 minutes with theaverage current density 3 Å/dm² in a plating solution that does notinclude iron ions.

Sample 2 indicates the plating performed for 33.3 minutes with theaverage current density 3 Å/dm² in a plating solution that includes ironions by 15 g/L.

Sample 3 indicates the plating performed for 33.3 minutes with theaverage current density 3 Å/dm² in a plating solution that includes ironions by 0.1 g/L.

FIG. 6 shows the results of the measurement of the degrees of roughnessof the plated surfaces of Samples 1 through 3 by using a roughness meterof a touch needle type.

In this figure, the average value of the degree of roughness of theplated surface of Sample 1, for which the pulse reverse electrolyticplating is performed for 33.3 minutes with the plating solution thatdoes not include iron ions, is 3.496 μm, whereas the average value ofthe degree of roughness of the plated surface of Sample 3, for which thepulse reverse electrolytic plating is performed for 33.3 minutes withthe plating solution that includes iron ions by 0.1 g/L, is 2.830 μm.Namely, it can be verified that a smoother plated surface can beobtained by Sample 3 for which the plating is performed with the platingsolution including iron ions.

Additionally, the average value of the degree of roughness of the platedsurface of Sample 2, for which the pulse reverse electrolytic plating isperformed for 33.3 minutes with the plating solution that includes ironions by 15 g/L is 1.821 μm, and a further smoother plated surface thanthat with the plating solution which includes iron ions by 0.1 g/L canbe obtained.

FIG. 7A shows the cross-sectional view of a microvia when the pulsereverse electrolytic plating is performed with the plating solutionwhich includes iron ions by 15 g/L under the above described conditions.In the meantime, FIG. 7B shows the cross-sectional view of a microviawhen the pulse reverse electrolytic plating is performed by using theplating solution which includes iron ions by 0.1 g/L.

Furthermore, FIG. 8 shows the cross-sectional view of a microvia whenthe pulse reverse electrolytic plating is performed with the platingsolution which does not include iron ions.

Note that a microvia hole tapers, the diameter of the aperture of thehole is 40 μm, the diameter of the bottom of the hole is 25 μm, and thedepth is 50 μm.

If the pulse reverse electrolytic plating is performed for 33.3 minuteswith the plating solution which includes iron ions by 15 g/L, themicrovia hole is completely filled up, and the copper plated surface issmooth as shown in FIG. 7A. Since a cavity in the middle of the microviais smaller in comparison with its depth (50 μm), it does not matterpractically.

If the pulse reverse electrolytic plating is performed for 33.3 minuteswith the plating solution which includes iron ions by 0.1 g/L, themicrovia hole is completely filled up a shown in FIG. 7B. Although thecopper plated surface is slightly rougher than that in FIG. 7A, it is alevel which does not matter practically.

FIG. 8 shows the cross-sectional view of a microvia hole when the pulsereverse electrolytic plating is performed with the plating solutionwhich does not include iron ions under the above described conditionsfor comparison. In this case, the microvia is filled up, but the copperplated surface becomes rougher in comparison with the case where theplating is performed with the plating solution which includes iron ionsby 0.1 g/L in FIG. 7B. If a plated surface is rough, a pattern on thelower surface of a resist is scraped when the resist is formed to etchthe pattern, leading to an unevenness of the width of the pattern.Therefore, it can be verified that a wiring pattern of higher qualitycan be obtained with the plating solution which includes iron ions inFIG. 7A or 7B.

It can be verified from the results of the comparisons between thedegrees of roughness of the plated surface and the cross-sectional viewsof the microvias in FIGS. 7A, 7B, and 8 that microvias can be filled upfor a time (approximately 33.3 minutes) shorter than a conventionalmethod by adding iron ions to a plating solution by 0.1 g/L or more, andby performing pulse reverse electrolytic plating, and at the same time,a plated surface can be formed to be smooth. Furthermore, judging fromthe result in the case where the plating is performed with the platingsolution which does not include iron ions under the same conditions, itcan be verified that an effect of smoothing a plated surface by addingiron ions is high.

According to the above described preferred embodiment, iron ions areadded to a copper dissolved solution and pulse reverse electrolyticplating is performed, whereby microvia holes can be filed up for a shorttime, and its surface can be almost smoothed.

Additionally, a plating solution is made to flow in parallel to thesurface to be plated (the surface on which microvia holes are formed) ofa printed-circuit board being a cathode, thereby further improving theplating characteristic. Furthermore, by controlling the flow quantity ofa plating solution to be a suitable value, the deposit speed of thesurface of the cathode 23 and that of copper within a microvia hole mybe set to a desired value.

In the above described preferred embodiment, “Fe²⁺”, is added to acopper plating solution. However, the present invention is not limitedto “Fe²⁺”, and other oxidization-reduction compounds may be added. Thepresent invention may also be applied to metal plating other thancopper. Furthermore, the application time of a forward/reverse platingcurrent, the current density of an electrode, the composition of aplating solution, a plating time, etc. are not limited to thoseimplemented in the above described preferred embodiment. For example,any composition can be used if it is available to electrolytic platingof copper, and other metals.

Still further, the direction in which a plating solution is made to flowupward or downward, not limited to the right and the left. Theessentiality is to make a plating solution flow in parallel to thesurface desired to be plated of a printed-circuit board. The presentinvention may be applied to a multi-layer substrate on which asemiconductor device is mounted, etc., not limited to a multi-layerprinted circuit board.

According to the present invention, microvia holes are filled up for ashort time, and a metal film having a smooth surface characteristic canbe formed. Namely, microvias which electrically connect wiring layersabove and below an insulation layer can be formed for a short time,thereby significantly reducing the manufacturing cost of a multi-layerwiring board.

1-6. (canceled)
 7. An electrolytic plating device, comprising; a wiringboard with microvia holes, each having a bottom made of copper foilprovided on a surface of the wiring board as an electrode; an insolubleelectrode, which is an electrode opposed to the wiring board; a metalplating solution containing iron ions of at least 0.1 gram/liter; apower source for performing electrolytic plating by applying aforward/reverse current between the wiring board and said insolubleelectrode; and a stirring unit stirring said metal plating solution tomake the solution flow in parallel to the surface to be plated of saidwiring board so that the microvia holes having the copper foil at thebottom, which are formed on the surface of said wiring board, may befilled up with said metal plating.
 8. The electrolytic plating deviceaccording to claim 7, wherein: the metal plating solution is comprisedof copper plating solution; and the stirring unit adjusts a flow rate ofthe copper plating solution to a level at which copper deposition speedsboth on the surface and inside microvia holes of the wiring board areoptimum.
 9. The electrolytic plating device according to claim 8,wherein the string unit adjusts the flow rate of the copper platingsolution to bring the iron ion amount present near to wiring boardsurface to a level at which all the microvia holes are almost fullyfilled and the plating layer thickness on the wiring board surfacebecomes optimum.
 10. The electrolytic plating device according to claim9, further comprising: a plating bath accommodating the insolubleelectrode and the wiring board; and a copper dissolved bath supplyingcopper ions to said plating bath, wherein said stirring unit circulatesa solution within the copper dissolved bath and the plating solutionwithin the plating bath.
 11. The electrolytic plating device accordingto claim 7, wherein: said insoluble electrode is implemented by amulti-aperture electrode; and said plating solution is implemented by acopper plating solution.